1. Field of the Invention
This invention relates to a waveform-forming device which is-equipped with a waveform memory storing waveform sample data.
2. Prior Art
Conventionally, waveform-forming devices which are equipped with a waveform memory storing digitalized data of waveform samples of musical tones are used in PCM tone generators of electronic keyboard instruments and the like. The digital data of waveform samples which are stored in the waveform memory are prepared by sequentially sampling waveforms of a predetermined musical tone at given sampling points, and converting values of amplitude of the waveforms sampled at respective sampling points into digital data (the digital data is hereinafter referred to as "a waveform sample"). A musical tone is reproduced by sequentially reading out the waveform samples corresponding to the musical tone from the waveform memory. To make efficient use of the waveform memory, the amplitude value of the waveform sample at each sampling point is not directly digitized into a so-called PCM format, but is formed into a compressed data format. Various types of compressed data format have already been proposed.
Conventionally proposed compressed data-forming methods include a so-called DPCM method which comprises determining a difference between each waveform sample and the following waveform sample and storing the determined difference into the waveform memory. According to the DPCM method, first, a waveform sample of non-compressed type (hereinafter referred to as "non-compressed waveform sample") is stored, and then a difference between each stored waveform sample and the following waveform samples is sequentially stored into the waveform memory, to thereby obtain compressed waveform samples. In reproducing waveform samples before compression from the compressed waveform samples, first, the non-compressed waveform sample is directly reproduced, then the non-compressed waveform sample and a first one of the above-mentioned differences stored are added together to reproduce a second waveform sample, and the second waveform sample thus reproduced and a second one of the differences are added together to reproduce a third waveform sample, and so forth. Thus, waveform samples are reproduced by adding together each waveform sample to be reproduced and the immediately preceding one reproduced.
More specifically, waveform sample data are read from the waveform memory in the following manner: First, frequency information or data on the frequency (what is called "F number") corresponding to the pitch of a musical tone to be produced is sequentially cumulated or counted by an accumulator to sequentially output a count value qF (q=1, 2, 3 . . . ). The integer part I of the count value is used as an address for access to each waveform sample to read data from the waveform memory at a predetermined frequency. If the predetermined sampling frequency is constant irrespective of waveforms of a musical tone to be reproduced, the count value qE has a positive real number having a decimal fraction d. Therefore, a waveform reproduced from data read out by using only each integer part as an address is not faithful to a desired waveform of the musical tone. To overcome this inconvenience, a plurality of waveform samples adjacent or close to a waveform sample at an address location of the waveform memory corresponding to the integer part I are read from the waveform memory, and an interpolation is carried out on the waveform samples by the use of the decimal fraction d to reproduce the interpolated waveform samples.
However, waveforms of a musical tone which are different only in pitch from waveforms stored in the waveform memory cannot be reproduced by reading out waveform sample data from every address in the waveform memory, but it is required to read waveform samples by skipping over every number of addresses corresponding to the pitch of the musical tone (skip access), as is so called "pitch-up" According to the conventional waveform-forming device described above, in which each waveform stored in the waveform memory is formed of one non-compressed waveform sample and a plurality of waveform samples indicative of differences between adjacent waveform samples, to form or reproduce a waveform, it is always required to add a waveform sample of the difference to an immediately preceding waveform sample data reproduced, starting with reproduction of a first non-compressed waveform sample This imposes a limitation upon the "pitch-up width" i.e. the amount by which the pitch of a musical tone to be formed from the waveform sample can be increased. Because of this limitation, it should be avoided to use the same waveform sample over a wide sound range by increasing the pitch-up width, but it is required to store lots of data of waveform samples of an identical timbre corresponding to respective different pitches by increasing the capacity of the waveform memory, which results in increased manufacturing costs.
Further, the limitation upon the pitch-up width also makes it impossible to follow up variations in the pitch due to,operation of a pitch envelope generator (EG) employed in the waveform-forming device.
An electronic musical instrument has been proposed by U.S. Pat. No. 4,916,996, which has a waveform memory storing compressed waveform data prepared by an ADPCM method, and wherein data are read from the waveform memory to generate a musical tone. According to the proposed musical instrument in repeatedly reading out a specific portion of waveforms of a musical tone, sample data reproduced from the start of the specific portion (which is obtained by converting differential sample data into linear sample data) is storied, and subsequently repeated reading-out is carried out by the use of the stored reproduced sample data as an initial value to thereby form the waveform of the musical tone.
However, the proposed electronic musical instrument also has the above described inconvenience that data cannot be read out from the waveform memory by skip access or "pitch-up".